Long-term OVRO monitoring of LS I +61°303: confirmation of the two close periodicities

¹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
e-mail: mmassi@mpifr-bonn.mpg.de; fjaron@mpifr-bonn.mpg.de
² Aalto University Metsähovi Radio Observatory, Metsähovintie 114, 02540 Kylmälä, Finland
e-mail: talvikki.hovatta@aalto.fi
³ Cahill Center for Astronomy and Astrophysics, California Institute of Technology, Pasadena CA, 91125, USA

Received: 12 January 2015
Accepted: 26 January 2015

Abstract

Context. The gamma-ray binary LS I +61°303 shows multiple periodicities. The timing analysis of 6.7 yr of GBI radio data and of 6 yr of Fermi-LAT GeV gamma-ray data both have found two close periodicities P_1,GBI = 26.49 ± 0.07 d, P_2,GBI = 26.92 ± 0.07 d and P_1,γ = 26.48 ± 0.08 d, P_2,γ = 26.99 ± 0.08 d.

Aims. The system LS I +61°303 is the object of several continuous monitoring programs at low and high energies. The frequency difference between ν₁ and ν₂ of only 0.0006 d^-1 requires long-term monitoring because the frequency resolution in timing analysis is related to the inverse of the overall time interval. The Owens Valley Radio Observatory (OVRO) 40 m telescope has been monitoring the source at 15 GHz for five years and overlaps with Fermi-LAT monitoring. The aim of this work is to establish whether the two frequencies are also resolved in the OVRO monitoring.

Methods. We analysed OVRO data with the Lomb-Scargle method. We also updated the timing analysis of Fermi-LAT observations.

Results. The periodograms of OVRO data confirm the two periodicities and .

Conclusions. The three independent measurements of P₁ and P₂ with GBI, OVRO, and Fermi-LAT observations confirm that the periodicities are permanent features of the system LS I +61°303. The similar behaviours of the emission at high (GeV) and low (radio) energy when the compact object in LS I +61°303 is toward apastron suggest that the emission is caused by the same periodically (P₁) ejected population of electrons in a precessing (P₂) jet.